The Pencil Project is a free and open source tool for making diagrams and GUI prototyping that everyone can use. Web designers and web application developers can use Pencil Project creating mockups for their websites or web applications easily. Pencil will always be free as it is released under the GPL version 2 and is available for virtually all platforms that Firefox 3 can run. Download it for free now.

Requirements:-
Demo: http://www.evolus.vn/Pencil/
License: GPL 2.0 License